Channel sounding method in wireless LAN system and device for supporting same

ABSTRACT

A method for channel sounding in a wireless local area network (WLAN) system and apparatus for the same is disclosed. An access point (AP) to perform the method for channel sounding broadcasts information on a restricted access window (RAW) for channel sounding included in a beacon, and transmits null data packets (NDP) for channel estimation in the RAW to a station (STA). The STA estimates channel information based on the NDPs, and transmits the estimated channel information to the AP.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 14/890,303,filed on Nov. 10, 2015 in the U.S. Patent and Trademark Office, which isa national stage application, filed under 35 U.S.C. § 371, ofInternational Patent Application No. PCT/KR2014/004214, filed on May 12,2014 and claims priority to and the benefit of Korean Patent ApplicationNo. 10-2014-0056368, filed on May 12, 2014, Korean Patent ApplicationNo. 10-2013-0055733, filed on May 16, 2013, and Korean PatentApplication No. 10-2013-0053442, filed on May 10, 2013 in the KoreanIntellectual Property Office, the entire contents of which areincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a wireless local area network (WLAN)system, and more particularly, to a method for channel sounding in aWLAN system.

BACKGROUND ART

A local area network (LAN) is largely classified into a wired LAN and awireless LAN (WLAN). The WLAN is a communication method performed in anetwork using an electric wave rather than a cable. With an advent ofthe WLAN, anticipation is growing for the WLAN to resolvecabling-related issues, such as installation, maintenance, or movementof cables. Moreover, as a number of mobile users are increasing, ademand for the WLAN is also set to surge.

A configuration of the WLAN includes an access point (AP) and a station(STA). The AP refers to equipment for transmitting an electric wave toenable WLAN users within a transmission distance to connect to theInternet and use a network. The AP functions as a base station (BS) of amobile phone or a hub of a wired network.

A basic building block of an Institute of Electrical and ElectronicsEngineers (IEEE) 802.11 network is referred to as a basic service set(BSS). The IEEE 802.11 network includes an independent BSS in which STAsin a BSS perform direct communication with one another, aninfrastructure BSS in which an AP is used during a process in which anSTA performs communication with STAs internal or external to a BSS, andan extended service set (ESS) in which BSSs are connected to one anotherto expand a service area.

In a next generation WLAN system, an AP simultaneously transmits a dataframe to at least one STA in a multiple input multiple output (MIMO)pairing.

In the WLAN system, when an AP and/or STA transmits a frame to a targetreceiver AP and/or STA, information on a channel to be used may beobtained through channel sounding. A process in which a transmitterrequests, from a receiver, channel information to be used for frametransception, and the receiver estimates a channel and sends a feedbackon the channel information to the transmitter is performed prior to theframe transception.

DISCLOSURE OF INVENTION Technical Solutions

According to an aspect of the present invention, there is provided amethod for channel sounding in a wireless local area network (WLAN)system performed by an access point (AP), the method includingbroadcasting a beacon including information on a restricted accesswindow (RAW) for channel sounding, and transmitting a plurality of nulldata packets (NDP) in the RAW to a station (STA).

The AP may control data transmission by the STA over a duration of theRAW for the channel sounding.

The AP may control the data transmission by the STA to be prohibitedover the RAW duration for the channel sounding.

The AP may control transmission of the channel information to the AP tobe allowed, and other transmissions to be prohibited, subsequent to thetransmission of the NDPs in the RAW duration for the channel sounding.

The beacon may include at least one of a bit value indicating whetherthe channel sounding is performed through the RAW, a bit value tocontrol the transmission by the STA in the RAW duration, and a bit valueindicating a communication type to be performed by the AP.

The method channel sounding may further include receiving, from the STA,channel information estimated based on the NDPs.

According to an aspect of the present invention, there is provided amethod for channel sounding in a WLAN system performed by an STA, themethod including receiving, from an AP, a beacon including informationon an RAW for channel sounding, estimating channel information based onan NDP received from the AP, and transmitting the estimated channelinformation to the AP.

The estimating may include identifying a communication type to beperformed by the AP based on the received beacon, receiving the NDP fromthe AP in the RAW when the STA supports the communication type, andestimating channel information based on the received NDP and theidentified communication type.

The estimating may include searching for a frequency channel thatsatisfies a predetermined reference from among frequency channels usedin communication with the AP, and determining information on the foundfrequency channel to be channel information.

The estimating may include searching for a sector to which the STAbelongs from among a plurality of sectors set by the AP, and determiningidentification information on the found sector to be channelinformation.

According to an aspect of the present invention, there is provided awireless apparatus including a transceiver to transmit and receive aframe, and a processor functionally associated with the transceiver,wherein the processor broadcasts a beacon including information on anRAW for channel sounding, and transmits a plurality of NDPs in the RAWto an STA.

According to an aspect of the present invention, there is provided awireless apparatus including a transceiver to transmit and receive aframe, and a processor functionally associated with the transceiver,wherein the processor receives, from an AP, a beacon includinginformation on an RAW for channel sounding, estimates channelinformation based on an NDP received from the AP, and transmits theestimated channel information to the AP.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a configuration of a wireless localarea network (WLAN) system.

FIGS. 2 and 3 are diagrams illustrating an uplink channel accessprotocol through allocation for a plurality of durations of wirelesscommunication.

FIGS. 4 through 6 are diagrams illustrating a method for sectorized beamoperation.

FIG. 7 is a diagram illustrating a method for channel sounding forsectorized beam operation according to an embodiment of the presentinvention.

FIG. 8 is a diagram illustrating an example of a method for channelsounding in sub-channel selective transmission (SST).

FIG. 9 is a diagram illustrating a method for channel sounding for SSTaccording to an embodiment of the present invention.

FIG. 10 is a diagram illustrating single user-multiple input multipleoutput (SU-MIMO) beamforming or multi user-MIMO (MU-MIMO) beamforming.

FIG. 11 is a diagram illustrating a method for channel sounding forSU-MIMO beamforming or MU-MIMO beamforming according to an embodiment ofthe present invention.

FIG. 12 is a flowchart illustrating an operation of a method for channelsounding performed by an access point (AP) according to an embodiment ofthe present invention.

FIG. 13 is a flowchart illustrating an operation of a method for channelsounding performed by a station (STA) according to an embodiment of thepresent invention.

FIG. 14 is a diagram illustrating a configuration of a wirelessapparatus according to an embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, reference will now be made in detail to embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. The embodiments are described below in order to explain thepresent invention by referring to the drawings. While this invention hasbeen particularly shown and described with reference to preferredembodiments thereof, it will be understood by those skilled in the artthat various changes in form and details may be made therein withoutdeparting from the spirit and scope of the invention as defined by theappended claims. The preferred embodiments should be considered indescriptive sense only and not for purposes of limitation. Therefore,the scope of the invention is defined not by the detailed description ofthe invention but by the appended claims, and all differences within thescope will be construed as being included in the present invention.

The following embodiments are combined in the form prescribed by thecomponents and features of the present invention. The elements orfeatures should be considered optional unless otherwise mentioned. Theembodiments may be carried out for each component or feature not beingcombined with other elements or features. In addition, it is alsopossible to configure embodiments herein by combining some of thecomponents and/or features. The order of operations described in theembodiments of the present disclosure can be changed. Some features orconfigurations of one embodiment may be included in another embodiment,or may be replaced with the corresponding features or configurations ofanother embodiment.

These specific terms are provided to assist understanding of thespecific terminology used in the following description of the presentinvention, and can be changed to another form without departing from thescope the technical idea of the present invention.

In some cases, the known structures and devices are omitted to avoidobscuring the concepts of the present invention, or illustrated in aform of a block diagram of the center of the core functionality of thestructures and devices.

The embodiments of the present invention can be supported by at leastone disclosed in the standard documents of such wireless access systems,for example, an Institute of Electrical and Electronics Engineers (IEEE)802 system, a 3 good publication practice (GPP) system, 3GPP long-termevolution (LTE) and LTE-advanced (LTE-A) systems, and a 3GPP2 system. Inother words, those steps or parts not described to clearly reveal thetechnical idea of the present invention may be supported by the standarddocuments. All terms disclosed in this specification can be explained bythe standard documents.

Technologies herein can be used for a variety of wireless accesssystems, such as code division multiple access (CDMA), frequencydivision multiple access (FDMA), time division multiple access (TDMA),orthogonal frequency division multiple access (OFDMA), and singlecarrier frequency division multiple access (SC-FDMA). CDMA can beimplemented in a radio technology such as universal terrestrial radioaccess (UTRA) or CDMA2000. TDMA can be implemented in a radio technologysuch as global system for mobile communications (GSM)/general packetradio service (GPRS)/enhanced data rates for GSM evolution (EDGE). OFDMAcan be implemented in a radio technology such as IEEE 802.11 (wirelessfidelity (Wi-Fi)), IEEE 802.16 (worldwide interoperability for microwaveaccess (WiMAX)), IEEE 802-20, and evolved-UTRA (E-UTRA). Hereinafter, anexample of the IEEE 802.11 system will be described for clarity, thoughnot limiting the scope the technical idea of the present inventionthereto.

FIG. 1 is a diagram illustrating a configuration of a wireless localarea network (WLAN) system.

The WLAN system includes at least one basic service set (BSS). As usedherein, the BSS may refer to a group of stations (STA) successfullysynchronized to communicate with one another, though not confined to aspecific area.

An infrastructure BSS includes at least one non-access point (AP) STA,an AP 110 to provide a distribution service, and a distribution systemto connect the AP 110 provided in multiple. In the infrastructure BSS,the AP 110 manages the non-AP STA of the BSS.

As used herein, the STA may refer to a predetermined function mediumincluding an IEEE 802.11-conformant medium access control (MAC) andphysical layer (PHY) interface to the wireless medium (WM) defined byIEEE 802.11. An example of the STA in a broad sense may include an APand a non-AP STA.

As used herein, the non-AP STA may refer to an STA not corresponding toan AP, hereinafter also referred to as a mobile terminal, a wirelessdevice, a wireless transmit/receive unit (WTRU), user equipment (UE), amobile station (MS), a mobile subscriber unit, or a user. Hereinafter,the non-AP STA is referred to as the STAs 120 for ease of description.

The AP 110 is a function object to provide connection with respect to adistribution system via a WM for the STAs 120 associated with acorresponding AP 110. In general, communication amongst the STAs 120 isperformed via the AP 110 in the infrastructure BSS including the AP 110.However, when a direct link is set amongst the STAs 120, at least onecorresponding STA 120 may communicate with one another directly, not viathe AP 110. For example, the AP 110 may be also referred to as a centralcontroller, a base station (BS), a node-B, or a base transceiver system(BTS).

A plurality of infrastructure BSSs including the BSS illustrated in FIG.1 is interconnected through the distribution system. The plurality ofinfrastructure BSSs interconnected through the distribution system isreferred to as an extended service set (ESS). The AP 110 and the STA 120included in the ESS communicate with one another, and the STA 120 movesto another BSS within the same ESS.

In the WLAN system pursuant to IEEE 802.11, a basic connection mechanismof MAC is a carrier sense multiple access with collision avoidance(CSMA/CA) mechanism. The CSMA/CA mechanism is also referred to as adistributed coordination function (DCF) of IEEE 802.11 MAC, andbasically adopts a “listen before talk” mechanism. For example, the AP110 and/or the STA 120 sense a wireless channel or a medium prior totransmitting a frame. When the medium is determined to be in an idlestatus based on a result of the sensing, the AP 110 and/or the STA 120commence transmitting the frame. Conversely, when the medium is detectedto be in an occupied status, the AP 110 and/or the STA 120 do notcommence transmission, and wait by setting a delay period for mediumaccess.

The CSMA/CA mechanism includes physical carrier sensing in which the AP110 and/or the STA 120 directly sense a medium, and virtual carriersensing. The virtual carrier sensing is conducted to remedy issues, forexample, a hidden node problem, likely to occur in medium access. MAC ofthe WLAN system uses a network allocation vector (NAV) for the virtualcarrier sensing. The NAV is a value indicated, by the AP 110 and/or theSTA 120 currently using or allowed to use a medium, of a period of timeremaining until the medium is available, to other AP 110 and/or the STA120. Accordingly, a value set as the NAV is related to a period of timeduring which the medium is to be used by the AP 110 and/or the STA 120that transmit a corresponding frame.

In the WLAN system of FIG. 1, the AP 110 transmits data to an STA groupincluding at least one STA from among the at least one STA 120associated with the AP 110.

In a multi user-multiple input multiple output (MU-MIMO) communicationsystem, the AP 110 transmits a plurality of space streams to the atleast one STA 120 using multiple antennas. When the AP 110 uses aplurality of transmission antennas, the AP 110 transmits data to the STA120 based on a beamforming technology to enhance a transmissionperformance.

The AP 110 requires channel information with respect to the STA 120 totransmit the data, and performs channel sounding to obtain the requiredchannel information. The channel sounding is performed based on a nulldata packet (NDP) and an NDP announcement (NDPA). The NDP includes aphysical layer convergence protocol (PLCP) protocol data unit (PPDU)format from which a data field in an MAC layer is excluded. The AP 110employs the NDP to extract the channel information from the STA 12. TheSTA 120 performs channel estimation based on a received NDP, and sends afeedback on channel status information to the AP 110 based on a resultof the estimation. For example, the STA 120 estimates an MIMO channelbased on very high throughput-long training fields (VHT-LTF) of the NDP,and obtains channel information. The NDP is also referred to as asounding frame.

The NDPA is transmitted to indicate an STA to receive the NDP, and alsoreferred to as a sounding announcement frame. The STA 120 determineswhether the STA 120 is an STA to participate in channel sounding basedon the NDPA. The AP 110 includes, in an NDPA frame, information on theSTA targeted for the channel sounding, and transmits the information tothe STA 120. The AP 110 instructs the STA 120 to receive the NDP usingthe NDPA frame. The AP 110 transmits the NDPA and the NDP based on arestricted access window (RAW).

The RAW is a duration of time in which access is permitted topredetermined STAs. The AP 110 prevents degradation of a channelsounding performance caused by an STA arbitrarily transmitting a frameduring a channel sounding process by transmitting the NDPA and the NDPbased on the RAW. The AP 110 optionally performs the channel soundingwith respect to predetermined STAs. Thus, the channel soundingperformance is enhanced.

FIGS. 2 and 3 are diagrams illustrating an uplink channel accessprotocol through allocation for a plurality of durations of wirelesscommunication.

Referring to FIG. 2, an AP controls an access of an STA through an RAW210. The RAW 210 includes a plurality of time slots. The STA wakes up ata target beacon transmission time (TBTT), and receives a beacon frame.The TBTT is a point in time at which the AP transmits the beacon frame.The beacon frame includes information on a slot duration for each RAW. Alength of a slot for each RAW is set differently. The STA determines achannel access slot set by the AP, and remains in a sleep status priorto the corresponding channel access slot. The STA commences channelaccess based on an enhanced distributed channel access (EDCA) protocoladjacent to a slot boundary of the corresponding channel access slot.

The AP reports the STA of whether a transmit opportunity (TXOP) schemeis applied to each RAW 210. For example, the AP indicates that a TXOP ortransmission in the TXOP does not deviate from a slot boundary. When theTXOP scheme is applied, the STA wakes up at the slot boundary, and doesnot wait for a period of Probe Delay.

Referring to FIG. 3, the AP sets a regulation for RAWs having differingslot lengths. “P” frame denotes a power saving poll (PS-Poll)/Triggerframe, “D” frame denotes a frame used in data transmission, and “A”frame denotes an acknowledgement (ACK) frame as shown in FIG. 3. Whenthe TXOP scheme is applied as the regulation for the RAWs havingdiffering slot lengths, a frame exchange sequence in the RAW may berestricted within a slot duration. The frame exchange sequence includesa start frame and a response frame. As shown in FIG. 3, an RAW1 310 isused as a protection duration for Poll signals, and an RAW2 320 is usedas a protection duration for DATA transmission.

FIGS. 4 through 6 are diagrams illustrating a method for sectorized beamoperation.

Referring to FIG. 4, an AP communicates with an STA based on the methodfor sectorized beam operation. In general, the method for sectorizedbeam operation is employed to re-use spatial resources in a networkinstalled in a wide area. An AP1 410 transmits data through sectorizedbeam transmission to an STAT 420. In a similar manner, an AP2 440transmits data through sectorized beam transmission to an STA2 460. Themethod for sectorized beam operation efficiently reduces interferencefrom neighboring APs and interference from an overlapped BSS (OBSS).Moreover, a hidden node issue in the same BSS may be mitigated because anumber of STAs in an activation status decreases when the method for thesectorized beam operation is used in a predetermined sector.

FIG. 5 illustrates an example of the method for the sectorized beamoperation. Referring to FIG. 5, an AP divides a space into a pluralityof sectors, and allocates resources to STAs belonging to each of theplurality of sectors based on a time division method. A beacon signal isindependently transmitted to each of the plurality of sectors, and eachof the STAs included in the sectors transmits or receives data within aduration of time allocated to each of the sectors. A predeterminedduration of time is allocated as a common duration of time available tobe used by the STAs in all of the sectors.

FIG. 6 illustrates another example of the method for the sectorized beamoperation. Referring to FIG. 6, an AP flexibly selects whether toperform sectorized beam transmission or omni-directional beamtransmission. Through this, an application of the method for thesectorized beam operation is relatively flexible. The omni-directionalbeam transmission is performed in RAW durations 610 and 620. Thesectorized beam transmission is allowed when a sector in a reservedshort duration of time is known to an AP, and available to be performedin the corresponding short duration of time. A sector reception beam isused in association with a sector transmission beam in a TXOP duration630. The AP indicates information on the sectorized beam transmissionthrough a beacon, a probe response, and an association response. Amethod of forming a sector beam may vary based on implementation plans.

FIG. 7 is a diagram illustrating a method for channel sounding forsectorized beam operation according to an embodiment of the presentinvention.

Referring to FIG. 7, an AP successively transmits a series of NDPs, orNDP frames, over a duration of an RAW for channel sounding, hereinafteralso referred to as a “sounding RAW.” The transmitted NDPs aretransmitted having short inter-frame spaces (SIFS) between each other.Each of the NDPs is transmitted to a corresponding signal for eachsector for sector training. FIG. 7 illustrates an example in which anumber of sector beams is four, and the AP performs channel soundingbased on an RAW or a periodic RAW (PRAW) subsequent to transmitting abeacon signal. A +high throughput control (+HTC) frame is transmittedprior to an NDP.

According to an embodiment of the present invention, transmissionperformed by an STA is prohibited during a sounding RAW to guarantee achannel sounding performance. However, the STA wakes up at a beginningof the sounding RAW, and listens to an entire sounding RAW. The channelsounding is performed on all STAs or STAs having a restricted range ofan association identification (AID). An STA targeted for the channelsounding is prohibited from transmission in a sounding RAW duration.

The AID refers to an identification (ID) assigned to an STA from an AP.

According to another embodiment of the present invention, anytransmission performed by an STA aside from transmission of a reportpacket to report a sector ID subsequent to receiving a series of NDPs isprohibited during a sounding RAW to guarantee the channel soundingperformance. The channel sounding is performed on all STAs or STAshaving a restricted range of an AID. The STA wakes up at a beginning ofthe sounding RAW, and listens to an end of the sounding RAW or an end ofthe transmission of the series of NDPs.

Due to such unique characteristics of an STA operation, sounding RAWindication information indicating whether an RAW is a sounding RAW or ageneral RAW is needed to be included in a beacon. The sounding RAWindication information is indicated on an RAW parameter set (PRS)information element (IE) transmitted from the beacon.

FIG. 8 is a diagram illustrating an example of a method for channelsounding in sub-channel selective transmission (SST).

An AP communicates with an STA through the SST. In the SST, the STAselects a sub-channel for transmission and reception performed by theSTA in a broadband BSS when allowed by the AP. The STA searches for anoptimum sub-channel for the communication with the AP, and transmitsinformation on the found optimum sub-channel to the AP. The AP indicatesrelated information through a beacon to enable STAs to measure channelsounding through frames transmitted subsequent to a beacon signal. Forexample, a beacon includes information on a number of signals, a type ofsignals, a frequency position of a signal, and a duration of time.

The AP optionally transmits a beacon indicating a sub-channel soundingsignal. The AP successively transmits a sounding signal of a singlechannel or multiple channels. The channel sounding is performed by anNDP signal, an additional beacon signal, or another packet by an AP.Information on a number of sounding signals, a type of sounding signals,for example, an NDP or a beacon, and a frequency position is included ina beacon. An NAV is set with respect to a sounding signal by a beacon.The AP selects a length of time allocated to a channel sounding process.The STA determines channels on which sounding is to be performed duringa period of an examining beacon over a plurality of cycles. The STAselects the optimum sub-channel based on a sounding signal. The STAdetermines whether to switch to additional channels based on beaconinformation. A beacon signal and a sounding signal are transmitted attime intervals of a point inter-frame space (PIFS).

FIG. 9 is a diagram illustrating a method for channel sounding for SSTaccording to an embodiment of the present invention.

Referring to FIG. 9, in the method for the channel sounding for SST, anAP successively transmits a series of NDPs. The NDPs are transmitted attime intervals of a PIFS, and each of the NDPs is transmitted withrespect to frequency channels allocated to the NDPs, respectively. Here,timing information of channel sounding using an NDP needs to beindicated in an RPS IE of a beacon. FIG. 9 illustrates an example inwhich a channel activity bitmap is {0, 0, 0, 0, 1, 1, 1, 1}, a maximumtransmission width is “2” megahertz (MHz), and a bandwidth span for eachNDP is 2 MHz. A bandwidth span of an NDPA may or may not be a duplicatemode.

According to another embodiment of the present invention, the method forthe channel sounding for SST is performed in conjunction with a PS-PollRAW. In an instance of downlink (DL) transmission, an STA to perform SSTdoes not transmit a PS-Poll frame with respect to an STA for which atraffic indication map (TIM) is set to be “1” directly subsequent toreceiving a beacon. When a PS-Poll duration pre-scheduled or newlyallocated for other purposes exists subsequent to a sounding RAW, theSTA waits until the corresponding PS-Poll duration terminates, andreports, to an AP, optimum sub-channel information obtained by aPS-Poll. The STA receives data from the AP through the correspondingoptimum sub-channel over an RAW duration for data transmission.

In an instance of uplink (UP) transmission, an STA to perform SSTtransmits data to an AP through an optimum sub-channel over an RAWduration for data transmission when the AP is informed through a PS-Pollwith a user data indication (UDI) of an amount of data to be transmittedover a PS-Poll RAW duration subsequent to a sounding RAW. When a frameto be transmitted to the AP exists, the STA transmits data to the APthrough the optimum sub-channel determined through channel sounding overa slot allocated to the RAW duration for the data transmission.

In the SST, a sounding RAW, a PS-Poll RAW, and an RAW for datatransmission need to be allocated subsequent to an activation timebecause the STA transmits data to the AP through a sub-channelsubsequent to the activation time defined in a beacon. Accordingly, thePS-Poll RAW in the DL transmission and UL transmission in the SST needsto be allocated subsequent to the sounding RAW. Although the PS-Poll RAWmay be allocated prior to the activation time, the PS-Poll RAW allocatedprior to the activation time corresponds to a PS-Poll RAW unrelated toan SST operation. Resources may be efficiently allocated by enabling theAP to identify optimum sub-channels for a plurality of STAs in advancewhile minimizing time resources consumed in the SST by allocating thePS-Poll RAW subsequent to the sounding RAW.

FIG. 10 is a diagram illustrating single user-MIMO (SU-MIMO) beamformingor MU-MIMO beamforming.

In 802.11 ac, the SU/MU-MIMO beamforming is defined as a combination ofan NDP announcement, an NDP, a feedback report, a beamforming reportpoll, and an additional feedback report. In a conventional SU/MUbeamforming method, information on STAs related to the SU/MU-MIMObeamforming is provided at a beginning of NDP sounding subsequent toreceiving an NDP announcement packet. The STAs not performing a powersaving mode and remaining awake to receive the NDP announcement packetis inappropriate for a device in which power saving is vital. An APneeds to provide information to the STAs on the NDP sounding using anRPS IE of a beacon in advance to control the STAs to wake up at thebeginning of the NDP sounding.

FIG. 11 is a diagram illustrating a method for channel sounding forSU-MIMO beamforming or MU-MIMO beamforming according to an embodiment ofthe present invention.

The method for the channel sounding for SU/MU-MIMO will be described bybeing divided into two embodiments.

<Method of Performing NDP Sounding for SU/MU-MIMO Beamforming Accordingto One Embodiment>

Referring to FIG. 11, a plurality of channel feedback reports is set tobe an additional RAW indicated in an RPS IE of a beacon subsequent to asounding RAW. Hereinafter, an RAW for transmission of feedback reportsis referred to as a report RAW. STAs wake up from a power saving mode ata beginning of a report RAW. An AID range of an STA targeted for channelsounding is indicated in the RPS IE of the beacon. A sequencecombination of STAs targeted for channel sounding is indicated in anNDPA provided in a predetermined sequence. An AP optionally allocates anRAW for a PS-Poll prior to the report RAW because the AP does not knowan amount of data of a channel feedback report to be transmitted by eachof the STAs and a feedback transmission mode for each of the STAs. ThePS-Poll describes UDI information indicating a data amount of a bufferfor a modulation and coding scheme (MCS) and UL transmission. The APoptionally allocates PS-Poll RAWs allocated to an additional slot for aplurality of STAs prior to the report RAW. Start timing information ofthe PS-Poll RAW is included in the RPS IE of the beacon. “P” framedenotes a PS-Poll frame, “D” frame denotes an UL data frame transmittedby an STA to an AP, and “A” frame denotes an ACK frame as shown in FIG.11.

<Method of Performing NDP Sounding for SU/MU-MIMO Beamforming Accordingto Another Embodiment>

In the method of performing the NDP sounding for the SU/MU-MIMObeamforming according to another embodiment, a beamforming RAW is set byextending a scope of a sounding RAW. An entire protocol sequence withrespect to various combinations of NDPA transmission, NDP transmission,a beamforming report feedback, a beamforming report poll, and anadditional beamforming report feedback is protected by the beamformingRAW. The beamforming RAW is set to be an additional RAW indicated in anRPS IE of a beacon, and classified as a type of a modified version ofthe sounding RAW. Transmission by an STA aside from transmission of achannel feedback report is prohibited during the beamforming RAW.Identifying a type of the beamforming RAW may be performed through asounding RAW indication and a sounding RAW type indication included inthe RPS IE of the beacon. Descriptions pertaining to the sounding RAWindication and the sounding RAW type indication will be provided later.

STAs wake up from a power saving mode at a beginning of the beamformingRAW. An AID range of an STA targeted for channel sounding is indicatedin the RPS IE of the beacon. A sequence combination of STAs targeted forchannel sounding is indicated in an NDPA in a predetermined sequence.Alternatively, the STAs wake up from the power saving mode at a slotboundary predetermined with respect to the STAs by additionally settingindependent slots respectively for combinations of an NDPA, an NDP, abeamforming report, a beamforming report poll, and an additionalbeamforming report in the beamforming RAW. Through this, relativelyefficient power saving may be possible.

An AP estimates a duration of time in which a channel feedback report istransmitted based on an MCS and a bandwidth pre-set to transmit thechannel feedback report, and based on a result of the estimation, sets alength of an entire beamforming RAW to be sufficiently long because theAP does not know an amount of data of the channel feedback report to betransmitted by each of the STAs and a feedback transmission mode foreach of the STAs. For example, the AP uses a basic rate of a BSS or anMCS in which an NDPA is transmitted as the transmission MCS for thechannel feedback report. The transmission bandwidth for the channelfeedback report is determined to correspond to a bandwidth of an NDPAsignal or an NDP signal.

FIG. 12 is a flowchart illustrating an operation of a method for channelsounding performed by an AP according to an embodiment of the presentinvention.

A method for channel sounding for SST and a method for channel soundingfor sectorized beam operation may be integrated because an STA operatesin the channel sounding for SST and the channel sounding for sectorizedbeam operation in a similar manner during a sounding RAW. For example,the STA may be prohibited from data transmission to stably performchannel sounding during the sounding RAW. The channel sounding isperformed on all STAs or STAs having a restricted range of an AID. TheSTA targeted for the channel sounding wakes up at a beginning of thesounding RAW, and listens to an entire sounding RAW. The integratedchannel sounding method may enable efficient use of spatial resourcesand frequency resources, and enhance a channel sounding performance.

Alternatively, any transmission by an STA aside from transmission of areport packet to report information on a sector ID or a selectedsub-channel subsequent to receiving a series of NDPs is prohibitedduring the sounding RAW to guarantee the channel sounding performance.The channel sounding is performed on all STAs or STAs having arestricted range of an AID. The STA wakes up at a beginning of thesounding RAW, and listens to the sounding RAW until an end of thesounding RAW or until the transmission of the series of NDPs terminates.

In the channel sounding for SST, the sounding RAW may be provided in aform of sweeping a plurality of frequency channels, or a form in whichtransmission of a report packet of information on a sub-channel selectedsubsequently is included. In the channel sounding for sectorized beamoperation, the sounding RAW may be provided in a form of sweeping aplurality of sectors, or a form in which transmission of a report packetrelated to information on a sector ID selected subsequently is included.

In operation 1210, an AP broadcasts a beacon including information on anRAW for channel sounding. The AP controls data transmission by an STA inan RAW duration for channel sounding. In an example, the AP controls thedata transmission by the STA to be prohibited in the RAW duration forchannel sounding. In another example, the AP controls transmission ofestimated channel information to the AP to be allowed, and othertransmission to be prohibited in the RAW duration for channel soundingsubsequent to transmission of NDPs.

The beacon transmitted by the AP includes at least one of a bit valueindicating whether channel sounding is performed through a correspondingRAW, a bit value to control transmission by an STA in a correspondingRAW duration, and a bit value indicating a communication type to beperformed by a corresponding AP.

According to an embodiment of the present invention, informationprovided in Table 1 is included in an RPS IE transmitted by the beacon.

TABLE 1 Feature Value Interpretation Page ID TBD bits Indicates the pageindex for hierarchical AID (based on hierarchical AID) of the allocatedgroup Block Offset TBD bits Assuming 32 blocks per page, these bitsindicate the starting block index of the allocated group Block Range TBDbits Indicates the number of blocks (starting from the block offset) forthe allocated group RAW Start 8 bits Duration in TU from end of beacontransmission to Time RAW Start time RAW Duration TBD bits Duration ofRAW in TU Access 2 bits Bit 1: Set to 1 if only STA with their TIM bitset to 1 restricted to are allowed to perform UL transmissions paged STAonly Bit 2: Set to 1 if RAW is reserved for frames with duration smallerthan slot duration, such as PS-Polls/ trigger frames (ignored if Bit 1is not set) Group/Resource 1 bits Set to 1 to indicate if STAs need towake up at the allocation frame beginning of the RAW to receive groupaddressed indication frames such as resource allocation (format of theresource allocation frame TBD) Sounding 1 bits Set to 1 to indicate ifnon-AP STAs are prohibited RAW to transmit but may elect to listen theentire RAW Indication Set to 0 otherwise Sounding TBD bit Set to 0 toindicate this Sounding RAW in for SST RAW Type sounding only for SSTcapable STAs Indication Set to 1 to indicate this Sounding RAW is forsector sounding only for sector capable STAs Set to 2 to indicate thisSounding RAW is for beamforming (SU/MU) only for beamformee capable STASOthers: TBD Slot definition TBD bits Include Slot duration signalingSlot assignment to STA Cross boundary transmissions allowed/not allowedFormat is TBD

As described in Table 1, the beacon includes a bit value indicatingwhether to prohibit transmission by STAs in a sounding RAW. The beaconincludes a bit value indicating whether a corresponding sounding RAW isused for the channel sounding for SST, the channel sounding forsectorized beam operation, or the channel sounding for SU/MU MIMObeamforming.

According to another embodiment of the present invention, informationprovided in Table 1 is included in an RPS IE transmitted by the beacon.

TABLE 2 Feature Value Interpretation Page ID TBD bits Indicates the pageindex for hierarchical AID (based on hierarchical AID) of the allocatedgroup Block Offset TBD bits Assuming 32 blocks per page, these bitsindicate the starting block index of the allocated group Block Range TBDbits Indicates the number of blocks (starting from the block offset) forthe allocated group RAW Start 8 bits Duration in TU from end of beacontransmission to Time RAW Start time RAW Duration TBD bits Duration ofRAW in TU Access 2 bits Bit 1: Set to 1 if only STA with their TIM bitset to 1 restricted to are allowed to perform UL transmissions paged STAonly Bit 2: Set to 1 if RAW is reserved for frames with duration smallerthan slot duration, such as PS-Polls/ trigger frames (ignored if Bit 1is not set) Group/Resource 1 bits Set to 1 to indicate if STAs need towake up at the allocation frame beginning of the RAW to receive groupaddressed indication frames such as resource allocation (format of theresource allocation frame TBD) Sounding 1 bits Set to 1 to indicate ifnon-AP STAs are prohibited to RAW transmit except but report packetsfollowing a series Indication of NDP but may elect to listen the entireRAW Set to 0 otherwise Sounding TBD bit Set to 0 to indicate thisSounding RAW in for SST RAW Type sounding only for SST capable STAsIndication Set to 1 to indicate this Sounding RAW is for sector soundingonly for sector capable STAs Others: TBD Slot definition TBD bitsInclude Slot duration signaling Slot assignment to STA Cross boundarytransmissions allowed/not allowed Format is TBD

As described in Table 2, the beacon includes a bit value indicatingwhether to prohibit other transmissions by STAs aside from transmissionof a report packet in a sounding RAW. The beacon includes a bit valueindicating whether a corresponding sounding RAW is used for the channelsounding for SST or the channel sounding for sectorized beam operation.

According to still another embodiment of the present invention,information provided in Table 3 is included in an RPS IE transmitted bythe beacon.

TABLE 3 Feature Value Interpretation Page ID TBD bits Indicates the pageindex for hierarchical AID (based on hierarchical AID) of the allocatedgroup Block Offset TBD bits Assuming 32 blocks per page, these bitsindicate the starting block index of the allocated group Block Range TBDbits Indicates the number of blocks (starting from the block offset) forthe allocated group RAW Start 8 bits Duration in TU from end of beacontransmission to Time RAW Start time RAW Duration TBD bits Duration ofRAW in TU Access 2 bits Bit 1: Set to 1 if only STA with their TIM bitset to 1 restricted to are allowed to perform UL transmissions paged STAonly Bit 2: Set to 1 if RAW is reserved for frames with duration smallerthan slot duration, such as PS-Polls/ trigger frames (ignored if Bit 1is not set) Group/Resource 1 bits Set to 1 to indicate if STAs need towake up at the allocation frame beginning of the RAW to receive groupaddressed indication frames such as resource allocation (format of theresource allocation frame TBD) Sounding 1 bits Set to 1 to indicate ifnon-AP STAs are prohibited to RAW transmit but may elect to listen theentire RAW Indication Set to 0 otherwise Sounding TBD bit Set to 0 toindicate this Sounding RAW in for SST RAW Type sounding only for SSTcapable STAs Indication Set to 1 to indicate this Sounding RAW is forsector sounding only for sector capable STAs Others: TBD Beamforming 1bits Set to 1 to indicate this Sounding RAW is for RAW beamforming(SU/MU) only for beamformee Indication capable STAS Set to 0 otherwiseSlot definition TBD bits Include Slot duration signaling Slot assignmentto STA Cross boundary transmissions allowed/not allowed Format is TBD

As described in Table 3, the beacon includes a bit value indicatingwhether to prohibit transmission by STAs in a sounding RAW. The beaconincludes a bit value indicating whether a corresponding sounding RAW isused for the channel sounding for SST or the channel sounding forsectorized beam operation. In addition, the beacon includes a bit valueindicating whether the corresponding sounding RAW is used for thechannel sounding for SU/MU MIMO beamforming.

According to further another embodiment of the present invention, acombination of a sounding RAW and a corresponding report RAW isrepresented in a form of an RAW type and a sub-mode as shown in Table 4.

TABLE 4 RAW Type RAW Type Bit 0 Bit 1 Description Options Subfield 0 0The RAW is a Generic RAW. Bit 0: Paged STA Bit 1: RA Frame 0 1 The RAWis a 0: SST Sounding Sounding/Report RAW. RAW 1: Sector Sounding RAW 2:SST Report RAW 3: Sector Report RAW 1 0 The RAW is a Simplex RAW. 0: APPM RAW 1: Non-TIM RAW 2: Omni RAW 3: Reserved 1 1 The RAW is aTriggering Reserved Frame RAW.

As described in Table 4, when the RAW type is a sounding RAW, a non-APSTA may not newly start a TXOP during an RAW, and select to listen tosector sounding or SST sounding. The Non-AP STA is allowed to transmit aresponse frame with respect to a frame transmitted by an AP during areceive priority control slot (RXP).

As described in Table 4, when the RAW type is a report RAW, a single ora plurality of report RAWs is scheduled by an AP in response to apreceding sounding RAW. In a corresponding RAW duration, a sector reportor an SST report is performed by a single or a plurality of STAs,irrespective of a setting of TIM bits corresponding to each of the STAs.

In the report RAW with respect to the sector sounding, the STA transmitsa sector report frame including a preferred sector ID to the AP at apoint in time not prior to a beginning of an assigned RAW slot. The APconfirms the aforementioned transmission of the sector report frame bytransmitting a response frame, for example, ACK, Block ACK, NDP ACK, orshort ACK, subsequent to an interval of an SIFS. In this example, theresponse frame transmitted by the AP includes information about aduration in which the corresponding STA maintains power saving. A lengthof a sector report frame exchange sequence may not exceed an assignedslot duration calculated by information provided in an “RAW SlotDefinition” subfield of an “RAW Assignment” field of an RPS element.

According to an embodiment of the present invention, an NDP PS Pollframe format is defined by Table 5 or Table 6 as a frame format withrespect to sector report frames. Table 5 represents an example of an MACframe body of an NDP PS Poll. Table 6 represents another example of anMAC frame body of an NDP PS Poll. The NDP PS Poll frame format expressedby Table 5 or Table 6 has a form substituted by a preferred sector IDvalue or an offset value with respect to a reference sector ID in a“Preferred MCS” field or an “UDI” field. Such a modified NDP PS Pollframe format is only available in the report RAW with respect to the SSTsounding or the sector sounding.

TABLE 5 Size Field (bits) Description NDP MAC 3 The NDP MAC Frame Typefield is set to 1 Frame Type RA 9 PARTIAL_AID addressed to AP TA 9PARTIAL_AID addressed to a STA Preferred TBD TBD MCS UDI 1 Se to 0: nouplink data Set to 1: uplink data present Reserved TBD

TABLE 6 Size Field (bits) Description NDP 3 The NDP MAC Frame Type fieldis set to 1 MAC Frame Type RA 9 PARTIAL_AID addressed to AP TA 9PARTIAL_AID addressed to a STA Preferred 4 Preferred MCS field indicatesthe preferred MCS level MCS of the STA for downlink transmission, andits value represents MCS index. This field may be used in determiningMCS level of PPDU for BU delivery. UDI 12 Set to 0: No uplink data Setto Non-zero: Duration of uplink data in unit of TU.

In the report RAW with respect to the SST sounding, the STA transmits anSST report frame including a preferred sub-channel index in a form of abitmap or coding to the AP at a point in time not prior to a beginningof an assigned RAW slot. The AP confirms the aforementioned transmissionof the SST report frame by transmitting a response frame, for example,ACK, Block ACK, NDP ACK, or short ACK, subsequent to an interval of anSIFS. In this example, the response frame transmitted by the AP includesinformation about a duration in which the corresponding STA maintainspower saving. A length of an SST report frame exchange sequence may notexceed an assigned slot duration calculated by information provided inan “RAW Slot Definition” subfield of an “RAW Assignment” field of an RPSelement.

According to another embodiment of the present invention, an NDP PS Pollframe format is defined by Table 5 or Table 6 as a frame format withrespect to SST report frames. Table 5 represents an example of an MACframe body of an NDP PS Poll. Table 6 represents another example of anMAC frame body of an NDP PS Poll. The NDP PS Poll frame format expressedby Table 5 or Table 6 has a form substituted by a preferred channelindex value or an offset value from a reference channel in a “PreferredMCS” field or an “UDI” field. Such a modified NDP PS Poll frame formatis only available in the report RAW with respect to the sector soundingor the SST sounding.

When the AP performs channel sounding, the AP schedules the channelsounding with respect to a plurality of STAs using an RAW of a beaconsignal. In this example, the AP sets a sounding RAW indication bit in anRPS IE to “1”, and sets a sounding RAW type indication to correspond toa type of the channel sounding to be performed. For example, thesounding RAW type indication indicates whether a corresponding soundingRAW is for channel sounding for SST or channel sounding for sectorizedbeam operation. A sounding RAW indication bit in an RPS IE of a beaconbeing set to “0” indicates that the channel sounding is not performed ina corresponding RAW. The sounding RAW may be transmitted based on aperiodic method or an arbitrary method.

In operation 1220, the AP transmits a plurality of NDPs in an RAW forchannel sounding. The AP successively transmits the plurality of NDPs atpredetermined time intervals. For example, the AP transmits the NPDs attime intervals of an SIFS or a PIFS. The STA wakes up at a beginning ofthe RAW for channel sounding, and remains awake until an end of thecorresponding RAW or the transmission of the series of the NDPsterminates.

When the AP performs channel sounding for SST, hereinafter also referredto as “SST sounding”, the AP transmits an NDPA in a sounding RAW, andsuccessively transmits a series of NDPs at intervals of a PIFS. In thisexample, the AP sets a sounding RAW type indication in the RPS IE tocorrespond to the SST sounding. The AP transmits the plurality of NDPsvia differing frequency channels, respectively. In this example, a basicunit of indicating an available frequency channel or a frequency channelfor transmission may change, and a bit field indicating an option withrespect to a basic unit setting may be defined. The available frequencychannel or the frequency channel for transmission with respect to anentire frequency band based on a corresponding basic unit may berepresented in a form of a bitmap or coding. The AP transmits the NDPsfor a plurality of frequency channels used for SST in the RAW for thechannel sounding. For example, the AP transmits a series of NDPsstarting from a frequency channel of a relatively low band at intervalsof a PIFS.

When the AP performs channel sounding for sectorized beam operation,hereinafter also referred to as “sector sounding”, the AP transmits anNDPA in a sounding RAW, and successively transmits a series of NDPs atintervals of a PIFS. In this example, the AP sets a sounding RAW typeindication in an RPE IE to correspond to sector sounding. The APtransmits the NDPs through differing sectors, respectively. The APtransmits the NDPs for the plurality of sectors, respectively, set bythe AP in an RAW duration for channel sounding. For example, the APtransmits a series of NDPs from a sector number “0” at time intervals ofa PIFS.

The AP receives channel information estimated based on an NDP from theSTA, and performs wireless communication with the ST based on thereceived channel information. The AP receives the channel informationthrough an RAW for a feedback report. The RAW for the feedback report isallocated subsequent to the RAW for channel sounding. In an instance ofthe channel sounding for SST, the AP receives optimum frequency channelinformation determined by the STA as channel information. In an instanceof the channel sounding for sectorized beam operation, the AP receivesoptimum sector ID information determined by the STA as channelinformation.

FIG. 13 is a flowchart illustrating an operation of a method for channelsounding performed by an STA according to an embodiment of the presentinvention.

In operation 1310, the STA receives a beacon including information on anRAW, for example, a sounding RAW, for channel sounding from an AP. TheSTA identifies a type of channel sounding through the beacon. Forexample, the STA determines whether the type of channel sounding is SSTsounding or sector sounding based on sounding RAW type informationincluded in an RPS IE of the beacon.

In operation 1320, the STA estimates channel information based on an NDPreceived from the AP. The NDP is transmitted in the RAW for channelsounding. For example, the channel information includes information onan optimum frequency channel for SST or on an optimum sector ID forsectorized beam operation. The STA is prohibited from data transmissionin an RAW duration for channel sounding. Alternatively, the STA isallowed to transmit the channel information to the AP subsequent to thetransmission of the NDPs, and prohibited from other transmission in theRAW duration for channel sounding. The STA identifies a communicationtype to be performed by the AP based on the beacon received from the AP,and determines whether the identified communication type is supported.When the identified communication type is supported, the STA receives anNDP from the AP in the RAW for channel sounding, and estimates thechannel information based on the received NDP or the identifiedcommunication type. For example, the communication type is one of SST,sectorized beam operation, SU-MIMO beamforming, and MU-MIMO beamforming.

According to an embodiment of the present invention, when the soundingRAW type is set to the SST sounding, the STA is prohibited from datatransmission during a sounding RAW. When the corresponding STA supportsan SST function, listening to an entire sounding RAW is allowed. The STAdetermines an optimum frequency channel based on an NDP. The STAsearches for a frequency channel that satisfies a predeterminedcriterion from among frequency channels used for communication with theAP, and determines information on the found frequency channel to bechannel information. In operation 1330, the STA transmits the estimatedchannel information to the AP. In one example, the STA reports thedetermined optimum frequency channel directly to the AP. In anotherexample, the STA reports information on the optimum frequency channelindirectly to the AP by transmitting data using the determined optimumfrequency channel. As such, the STA determines the frequency channelthat satisfies the predetermined criterion from among the plurality offrequency channels based on the NDP, and transmits the channelinformation to the AP using the determined frequency channel.Alternatively, the STA is prohibited from other transmissions aside fromthe reporting of the optimum frequency channel subsequent to receiving aseries of NDPs during the sounding RAW. When the corresponding STAsupports the SST function, or the transmission of the series of NDPsterminates listening to the entire sounding RAW is allowed. The STAtransmits the determined optimum frequency channel information to the APthrough a report packet. When each of multiple STAs sends a feedback onthe optimum frequency channel to the AP, the AP controls frames for thefeedback on the optimum frequency channel to be transmitted though thereport RAW to avoid a collision between the frames for the feedback onthe optimum frequency channel and other signals. In this example, abeginning of the report RAW and a series of information are included inthe RPS IE of the beacon transmitted by the AP. When the estimatedchannel information is transmitted to the AP, transmission by other STAsaside from an STA that transmits the channel information is prohibited.

According to another embodiment of the present invention, when thesounding RAW type is set to the sector sounding, the STA is prohibitedfrom data transmission during a sounding RAW, and when the correspondingSTA supports a sectorized beam operation function, listening to anentire sounding RAW is allowed. Alternatively, the STA is prohibitedfrom other transmission aside from reporting an optimum sector IDsubsequent to receiving a series of NDPs during the sounding RAW. Whenthe corresponding STA supports the sectorized beam operation function,or until the transmission of the series of NDPs terminates, listening tothe entire sounding RAW is allowed. The STA searches for a sector towhich the STA belongs from among a plurality of sectors set by the AP,and determines identification information of the found sector to bechannel information. In operation 1330, the STA transmits the estimatedchannel information to the AP. The STA optionally sends feedback on thedetermined optimum sector ID to the AP subsequent to sector sounding.The STA transmits the determined optimum sector ID to the AP through areport packet. When each of multiple STAs sends feedback on the optimumsector ID to the AP, the AP controls frames for the feedback on thesector ID to be transmitted though the report RAW to avoid a collisionbetween the frames for the feedback on the sector ID and other signals.In this example, a beginning of the report RAW and a series ofinformation are included in the RPS IE of the beacon transmitted by theAP. When the estimated channel information is transmitted to the AP,transmission by other STAs aside from an STA that transmits the channelinformation is prohibited.

FIG. 14 is a diagram illustrating a configuration of a wirelessapparatus 1410 according to an embodiment of the present invention.

Referring to FIG. 14, the wireless apparatus 1410 includes a processor1430, a memory 1440, and a transceiver 1420. The wireless apparatus 1410corresponds to an AP or an STA of the present invention.

The transceiver 1420 transmits or receives a wireless signal. Theprocessor 1430 is set to operate by being functionally connected to thetransceiver 1420. The processor 1430 controls the wireless apparatus1410 to perform the channel sounding method described with reference toFIGS. 7, 9, and 11 through 13.

At least one of the processor 1430 and the transceiver 1420 includes atleast one of an application-specific integrated circuit (ASIC), a logiccircuit, and a data processing apparatus. When embodiments areimplemented in software, the aforementioned method may be implemented ina module, a process, or a function that performs the functions describedin the foregoing. The module is stored in the memory 1440, and performedby the processor 1430. The memory 1440 may be included inside theprocessor 1430, and functionally connected to the processor 1430 bybeing additionally disposed external to the processor 1430 in variousknown manners.

In the exemplary system, the above-mentioned methods are described basedon the flowchart by a series of steps or blocks, but the presentinvention is not limited to the order of the steps, and some steps mayoccur in a different order or simultaneously with other steps. Also,those skilled in the art will be able to understand that the differentstages or steps shown in the flowchart are not exclusive but can befurther added, or one or more steps of the flowchart may be deletedwithout affecting the scope of the present invention.

The software may include a computer program, a piece of code, aninstruction, or some combination thereof, for independently orcollectively instructing or configuring the processing device to operateas desired. Software and data may be embodied permanently or temporarilyin any type of machine, component, physical or virtual equipment,computer storage medium or device, or in a propagated signal wavecapable of providing instructions or data to or being interpreted by theprocessing device. The software also may be distributed over networkcoupled computer systems so that the software is stored and executed ina distributed fashion. In particular, the software and data may bestored by one or more computer readable recording mediums.

The computer readable recording medium may include any data storagedevice that can store data which can be thereafter read by a computersystem or processing device. Examples of the computer readable recordingmedium include read-only memory (ROM), random-access memory (RAM),CD-ROMs, magnetic tapes, floppy disks, optical data storage devices.Also, functional programs, codes, and code segments for accomplishingthe example embodiments disclosed herein can be easily construed byprogrammers skilled in the art to which the embodiments pertain based onand using the flow diagrams and block diagrams of the figures and theircorresponding descriptions as provided herein.

A number of examples have been described above. Nevertheless, it shouldbe understood that various modifications may be made. For example,suitable results may be achieved if the described techniques areperformed in a different order and/or if components in a describedsystem, architecture, device, or circuit are combined in a differentmanner and/or replaced or supplemented by other components or theirequivalents.

Accordingly, other implementations are within the scope of the followingclaims.

The invention claimed is:
 1. A method for channel sounding in a wirelesslocal area network (WLAN) system performed by an access point (AP), themethod comprising: broadcasting a beacon comprising information on asub-channel selective transmission (SST) sounding restricted accesswindow (RAW) for channel sounding; and transmitting a plurality of nulldata packet (NDP) frames to a station (STA) through a plurality ofchannels in the SST sounding RAW, wherein each of the plurality of NDPframes indicates a channel allocated to each of the plurality of NDPframes, wherein a sub-channel is selected by the STA based oninformation included in the plurality of NDP frames, wherein theplurality of NDP frames are successively transmitted at predeterminedtime intervals of a short inter-frame space (SIFS), and wherein the SSTsounding RAW is allocated to the STA only for the channel sounding. 2.The method of claim 1, wherein the AP controls the data transmission bythe STA to be prohibited over the SST sounding RAW for the channelsounding.
 3. The method of claim 2, wherein the AP controls transmissionof channel information to the AP to be allowed, and other transmissionto be prohibited, subsequent to the transmission of the NDP frames inthe SST sounding RAW for the channel sounding.
 4. The method of claim 1,wherein the beacon comprises at least one of: a bit value indicatingwhether the channel sounding is performed through the SST sounding RAW,a bit value to control the transmission by the STA in a duration of theSST sounding RAW, and a bit value indicating a communication type to beperformed by the AP.
 5. The method of claim 1, wherein the transmittingcomprises: transmitting the NDP frames for a plurality of sectors set bythe AP in the SST sounding RAW for the channel sounding.
 6. The methodof claim 1, wherein the method further comprising: receiving, from theSTA, a report frame including information on at least one channelselected by the STA based on the NDP frames.
 7. The method of claim 6,wherein the receiving comprises: receiving the report frame through anadditional RAW for a feedback report, wherein the additional RAW for thefeedback report is allocated subsequent to the SST sounding RAW for thechannel sounding.
 8. The method of claim 1, wherein channel informationis estimated based on the NDP frames.
 9. The method of claim 1, whereinthe transmitting comprises: transmitting the NDP frames at predeterminedtime intervals of a point inter-frame space (PIFS).
 10. The method ofclaim 1, wherein the plurality of NDP frames are successivelytransmitted after a sounding announcement frame is transmitted to theSTA to indicate the STA to receive the NDP frames.
 11. A method forchannel sounding in a wireless local area network (WLAN) systemperformed by a station (STA), the method comprising: receiving, from anaccess point (AP), a beacon comprising information on a sub-channelselective transmission (SST) sounding restricted access window (RAW) forchannel sounding; receiving, from the AP, a plurality of null datapackets (NDP) frames through a plurality of channels in the SST soundingRAW for the channel sounding, wherein each of the plurality of NDPframes indicates a channel allocated to each of the plurality of NDPframes; estimating channel information based on the plurality of NDPframes received from the AP; and selecting a sub-channel based oninformation included in the plurality of NDP frames, wherein theplurality of NDP frames are successively transmitted at predeterminedtime intervals of a short inter-frame space (SIFS), and wherein theplurality of NDP frames are transmitted to the STA in the SST soundingRAW.
 12. A wireless apparatus comprising: a transceiver to transmit andreceive a frame; and a processor functionally associated with thetransceiver, wherein the processor broadcasts a beacon comprisinginformation on a sub-channel selective transmission (SST) soundingrestricted access window (RAW) for channel sounding, and transmits aplurality of null data packet (NDP) frames to a station (STA) through aplurality of channels in the SST sounding RAW, wherein each of theplurality of NDP frames indicates a channel allocated to each of theplurality of NDP frames, wherein a sub-channel is selected by the STAbased on information included in the plurality of NDP frames, whereinthe plurality of NDP frames are successively transmitted atpredetermined time intervals of a short inter-frame space (SIFS), andwherein the SST sounding RAW is allocated to the STA only for thechannel sounding.
 13. A wireless apparatus comprising: a transceiver totransmit and receive a frame; and a processor functionally associatedwith the transceiver, wherein the processor receives, from an accesspoint (AP), a beacon comprising information on a sub-channel selectivetransmission (SST) sounding restricted access window (RAW) for channelsounding, receives, from the AP, a plurality of null data packet (NDP)frames through a plurality of channels in the SST sounding RAW for thechannel sounding, wherein each of the plurality of NDP frames indicatesa channel allocated to each of the plurality of NDP frames, estimateschannel information based on the plurality of NDP frames received fromthe AP, and selects a sub-channel based on information included in theplurality of NDP frames, wherein the plurality of NDP frames aresuccessively transmitted at predetermined time intervals of a shortinter-frame space (SIFS), and wherein the plurality of NDP frames aretransmitted to the STA in the SST sounding RAW.